Currently, powder coating materials are used mainly for protecting metal substrates from rusting, exposure to solvents, and so on. In recent years, there has been a tendency to avoid the use of solvent type coating materials in order to reduce the negative impact on the environment. In addition, powder coating materials have the potential to increase productivity and reduce coating costs, compared to solvent type coating materials. For these reasons, the use of powder coating materials for the purposes other than those described above has been a focus of attention.
One example of such powder coating materials is a powder coating material containing a polyvinyl acetal powder. For example, JP 63(1988)-193970 A discloses an epoxy resin powder coating material containing 5 to 20 parts by weight of polyvinyl butyral or polyvinyl formal per 100 parts by weight of an epoxy resin that is a thermosetting resin, as well as a curing agent and a filler. In general, however, this powder coating material containing polyvinyl acetal is not necessarily good in flowability during the coating process. This powder coating material also does not necessarily allow a coating film formed therefrom to have excellent impact resistance nor excellent uniformity of thickness (uniformity of coating film thickness).
As another form of the powder coating material containing a polyvinyl acetal powder, there is a powder coating material containing a polyvinyl acetal powder and a pigment powder. As an example of such a powder coating material, a powder coating material obtained by melt-kneading a polyvinyl butyral powder and a pigment powder and pulverizing the melt-kneaded mixture are known (see, for example, JP 10(1998)-212433 A (Claims, and paragraphs [0010], [0011], [0023] and [0048]), JP 10(1998)-206392 A (Claims, and a paragraph [0011]), and JP 2000-281966 (Claims, and paragraphs [0026], [0037], [0039], [0062] and [0097])). However, for obtaining this type of a powder coating material, a process to melt-knead a polyvinyl butyral powder and a pigment as well as a process to pulverize not only unkneaded raw materials but also the melt-kneaded mixture are needed, which increases the manufacturing cost, resulting in poor productivity. In addition, the melt-kneaded mixture is pulverized by mechanical pulverization, and it is thus not easy to obtain desired small diameters of particles. Furthermore, it is also difficult to obtain particles almost spherical in shape.
As a method for preparing powder coating materials other than the method of melt-kneading a resin powder and a pigment as described above, a method of dry-blending a resin powder and a pigment is also known. With respect to such a powder coating material obtained by dry-blending, a powder coating material obtained by dry-blending at least two types of powder coating materials, each of which has been obtained by melt-kneading a resin powder and a pigment such as carbon black and pulverizing the melt-kneaded mixture, are known as a powder coating material for easy coloring and toning (see, for example, JP 7(1995)-188586 A (Claims, and paragraphs [0007], [0010], [0014] and [0015])). However, JP 7(1995)-188586 A discloses dry-blending between the melt-kneaded mixtures (primary color powder coating materials) of a resin powder and a pigment powder, but does not disclose dry-blending of a resin powder with a pigment powder to obtain a powder coating materials. JP 7(1995)-188586 A does not either disclose the use of a polyvinyl butyral powder as a resin component of a coating material.
As another powder coating material for easy coloring and toning obtained by dry-blending, a powder coating material obtained by dry-blending a resin powder with an average particle diameter of 5 to 50 μm and a pigment powder with an average particle diameter of 0.01 to 1.0 μm is also known (see, for example, JP 2004-43669 A (Claims, and paragraphs [0024], [0041], [0073], [0087] and [0088])). However, JP 2004-43669 A describes a butyral resin (polyvinyl butyral) just as an example of the resin powder, but gives no working example of the powder coating material. The present inventors have studied actually using polyvinyl butyral (see Comparative Examples to be described below in the present description), and have found that the use of a pigment powder with such a small average particle diameter causes a separation of the resin powder and the pigment powder into two layers, and results in poor coloration and insufficient flowability of the resulting powder coating material.
Meanwhile, as still another powder coating material, a polyamide powder coating material containing a polyamide powder is known. The polyamide powder coating material containing a polyamide powder has a feature of excellent impact resistance of a coating film formed on the surface of a substrate (hereinafter, simply referred to as a “coating film”). However, in order to secure the adhesion to the substrate, pre-treatment of the substrate surface such as primer coating is required. The polyamide powder coating material generally has a drawback of being poor in hardness of the coating film. Therefore, it is desired for the polyamide powder coating material to improve the adhesion to the substrate and the hardness of the coating film.
JP 10(1998)-212433 A discloses that at least two types of resin powders are blended to obtain a powder coating material and that a polyamide powder and a polyvinyl butyral powder can be used as resin powders to be blended. However, JP 10(1998)-212433 A neither discloses nor suggests the selective combined use of a polyvinyl butyral powder and a polyamide powder, the technical significance and effects of the combined use of these powders, and the combination ratios for the combined use thereof.